This invention relates generally to the field of photography and, more particularly, to an improved exposure control method and apparatus for evaluating different spectral frequencies from different zones of the scene to be evaluated.
Automatic light responsive exposure control systems are well-known in the photographic arts. One such automatic exposure control system employs scanning type shutter blades. Exemplary scanning shutter blades usable in such systems are disclosed generally in U.S. Pat. No. 3,942,183, issued Mar. 2, 1976, to George D. Whiteside; and U.S. Pat. No. 4,104,653, issued Aug. 1, 1978, to Bruce K. Johnson et al., all of which are presently assigned in common with the present application. As described in these patents, cooperating pairs of taking or primary, and secondary or photocell apertures are formed in the shutter blades. These aperture pairs cooperate respectively for blocking and unblocking scene light through an exposure opening to a film plane, and through a photocell opening to a light sensing device or photoresponsive cell used for controlling blade positioning. During the exposure cycle, the photocell apertures operate in conjunction with the photocell and a control circuit to define both the taking aperture values achieved and the exposure interval as a function of the amount of scene radiation received through the photocell apertures.
It will be understood that the typical photocell tends to react to a band of certain spectral frequencies which include infrared (IR) frequencies. This is especially so when the photocell is of the silicon type, because it tends to be red (IR) sensitive. Therefore, for optimizing photographic quality when using systems of the type noted, a spectral correction filter can be employed in the photocell's optical path. Such filter correlates the spectral sensitivity curve of the photoresponsive element more closely with that of the eye. Ordinarily then, use is made of a spectral correction filter having peak absorption in the near-infrared region and high transmission in the visual region.
While use of infrared spectral filters serve satisfactorily, complications can arise with its usage. This is so particularly in flash exposures when reflectivities of different objects to be photographed exhibit widely disparate values. Partly as a result of this, it has been found advantageous to evaluate infrared frequencies in flash exposures. Such approach is disclosed in copending and commonly assigned application Ser. No. 156,198, entitled "Method and Apparatus for Selectively Positioning Spectral Filter", filed June 3, 1980, by Bruce K. Johnson now U.S. Pat. No. 4,315,675, issued Feb. 16, 1982. In particular, an exposure control system is disclosed with artificial flash or strobe lighting associated with a pair of photoresponsive regions. One region evaluates infrared spectral frequencies for the strobe mode and another region evaluates visible spectral frequencies in the ambient mode. For effectuating such switching, the shutter mechanism is provided with a unique photocell aperture arrangement which, during shutter scanning, alternatively directs scene radiation to one or the other photoresponsive region depending on the ambient light conditions.
This approach represents a significant advance in providing improved exposure under many different lighting conditions. However, with this latter approach, the dynamics of the blade mechanism must be relatively accurately controlled for insuring the desired switching of the evaluation of different spectral frequencies. Also, with such a system, the blocking visible filter (i.e., passes infrared frequencies) is controlling passage of scene radiation to the photocell for a relatively significant portion of the time the photocell circuit is integrating the scene radiation. Hence, when the infrared frequencies are being evaluated, there is a tendency for underexposure, particularly when the scene contains plants, trees, grass, etc. Thus, it is desirable to limit, as much as possible, the time interval the infrared is being evaluated. Moreover, with the foregoing system it is highly desirable to accurately time firing of the flash when the infrared is being evaluated.
It is apparent, therefore, that the cooperation of the photocell apertures and the scanning dynamics of the blades as well as the timing of the flash firing must be accurately controlled. Otherwise, the desired switching of scene radiation evaluations for controlling exposure can be adversely affected.
Moreover, such light detecting devices of the above kind generally comprise one or more photovoltaic or photoresistive transducers associated with an optical system of the photocell. Such a light detecting device, however, reacts to the average brightness of a field of view. A difficulty with using the average field brightness is that it differs from the brightness of particular portions of the field, such as the subject of interest in most scenes. Hence, when the brightness of peripheral or upper zones of the field contrasts with the brightness of central or lower zones of the field, an exposure meter having a single photocell indicates an incorrect exposure for the more important subject usually in the center of the scene. Thus, a unicell photometer is less than satisfactory for certain photographic situations.
Approaches have been made for correcting the foregoing drawback. One is to employ a multicell photometer. Such a photometer evaluates and weighs the radiation from different zones of the scene to be evaluated and photographed.
Another approach is disclosed in commonly assigned and copending application Ser. No. 219,930, entitled "Light Detecting Apparatus for Photographic Camera", by Bruce K. Johnson, and filed Dec. 24, 1980 and now U.S. Pat. No. 4,345,828. In this approach, an intricate photocell aperture arrangement on the blades cooperate with a single photocell to evaluate at least partially different portions of the scene. In addition, essentially infrared evaluation from a given portion of the scene is used for flash contributions to the photograph whereas visible light evaluation from at least a partially different portion of the scene is used to control the ambient contribution. However, as with the exposure control system described in the previously noted application Ser. No. 156,198, the camera arrangement requires precise, intricate manipulation of the photocell aperture arrangement dynamically formed on the blades during scanning, as well as accurate timing of the firing of the flash. It becomes advantageous, therefore, to reduce the amount of time, labor and effort required to insure such intricate cooperation and timing.